(require 'interlist)

(defvar presentation-mode nil
  "Tells if `presentation-mode' is active in buffer.")
(defvar presentation-normal-keymap nil
  "The normal keymap used in this buffer.")
(defvar presentation-old-read-only nil
  "Old value of read-only flag for buffer.")

(make-variable-buffer-local 'presentation-mode)
(make-variable-buffer-local 'presentation-normal-keymap)

(putassq 'presentation-mode '(" OH") minor-mode-alist)


(defun presentation-scroll-up ()
  (interactive)
  (condition-case ()
      (scroll-up)
    (end-of-buffer
     (let ((saved-point (point)))
       (narrow-to-page +1)
       (if (= (point) saved-point)
	   (signal 'end-of-buffer nil))))))

(defun presentation-scroll-down ()
  (interactive)
  (condition-case ()
      (scroll-down)
    (beginning-of-buffer
     (let ((saved-point (point)))
       (narrow-to-page -1)
       (if (= (point) saved-point)
	   (signal 'beginning-of-buffer nil)
	 ;; XXX: I'm not sure which of the two behaviours below I want.
	 (if nil
	     (goto-char (point-min))
	   (goto-char (point-max))
	   (recenter -1)
	   (move-to-window-line 0)))))))

(defun presentation-mode (&optional arg)
  "Minor mode for displaying a file as a \"slide presentation\"."
  (interactive "P")
  (let ((new-presentation-mode-value (if arg
					 (> (prefix-numeric-value arg) 0)
				       (not presentation-mode))))
    (cond
     ;; Toggle from normal to presentation mode
     ((and new-presentation-mode-value (not presentation-mode))
      (setq presentation-old-read-only	buffer-read-only
	    buffer-read-only		t
	    presentation-normal-keymap	(current-local-map))
      (use-local-map (copy-keymap (or (current-local-map)
				      (make-sparse-keymap))))
      (local-set-key " "	#'presentation-scroll-up)
      (local-set-key [next]	#'presentation-scroll-up)
      (local-set-key "\177"	#'presentation-scroll-down)
      (local-set-key [prior]	#'presentation-scroll-down)
      (narrow-to-page)
      )

     ;; Toggle from presentation to normal mode
     ((and (not new-presentation-mode-value) presentation-mode)
      (setq buffer-read-only presentation-old-read-only)
      (use-local-map presentation-normal-keymap)
      (widen))

     ;; Remaining in either normal or presentation mode
     (t
      nil)
     )
    (setq presentation-mode new-presentation-mode-value)))